6BX7 10W bile duct amplifier

Recently, a friend in the south sent me some vacuum tubes and some "antique" resistors and capacitors that had been dormant for many years, and asked me to assemble an amplifier for him. After carefully identifying these blurred vacuum tubes with a magnifying glass, I finally identified them as ECL82, 6BX7 and 6x5. ECL82 is a triode and pentode tube, which was used for field oscillation in the tube TV sets in the 1960s. The triode part is a high μ voltage amplifier tube, and the pentode part is a power tube. 6BX7 is a dual triode power tube, which is used for field output in tube TV sets. 6X5 is a side-heated oxide cathode rectifier diode, which has basically the same parameters as the common 6Z4, except that the order of the pins is different.

The ECL82 triode is used as the preamplifier. Its plate load resistance is relatively large (270kΩ), and the gain is above 60dB, which fully meets the requirements of various modern sound sources. The pentode part is connected in the form of a triode, and is used for D-K split inversion to ensure sufficient driving voltage output under low load impedance. This inversion stage has about 5% negative feedback. The dynamic range, output level, and distortion are all very good. It is suitable for driving low μ, low internal resistance triode power output stage.

6BX7 is a high transconductance, low internal resistance dual triode power tube, with a small plate, suitable for AB1 class amplification. According to the characteristics of the plate voltage, when the plate voltage is 305v and the gate negative voltage is -28, the output power is about 10.5w in the AB1 class working state. The load impedance between the plate electrodes is 8kΩ, and the plate power consumption of each triode is 6w. About -10dB negative feedback is added to the whole circuit, which slightly reduces the input sensitivity.

Two power transformers are used, with a power of 70W. T1 is a high-quality silicon steel sheet with a tongue width of 26mm and a stack thickness of 45mm. The primary is wound with Φ0.29mm enameled wire for 825 turns, the secondary is wound with Φ0.21mm enameled wire for 1200 turns, the G1 and G2 filament voltages are wound with Φ1.08mm enameled wire for 24 turns, and the 6X5 filament voltage is supplied separately. T2 is a finished 30W secondary output 24V transformer, which outputs a 68X7 grid negative voltage of -28V after bridge rectification. Adjust W2 and w3 to output a stable negative voltage.

The coupling capacitor in the circuit and the 0.047μF mica capacitor at both ends are connected to help improve the treble.